Filtration is a process in which contaminants in a fluid such as water, for example, particulate matter and, in some cases, dissolved species, are removed from the fluid by flowing the contaminated fluid through a filter membrane. Filtration exists in two basic forms, dead-end filtration or cross-flow filtration. The defining aspect of dead-end filtration is that all of the liquid fed to the filter membrane (the “feed stream”) must pass through the filter membrane and all the filtered material must be retained by the filter membrane. A drip-type coffee maker, in which ground coffee is placed in a basket which has been lined with a filter membrane of porous paper, provides an example of dead-end particulate filtration. Hot water is poured onto the coffee grounds in the basket to constitute a feed stream for the filter. As the water passes through the coffee grounds, it dissolves the soluble components of the coffee grounds. The water and the soluble components therein pass through the filter membrane, while the insoluble components of the coffee remain in particulate form and are retained by the filter membrane. Filtration of this type has practical limits on the size of the particulate matter which can be removed and the amount of power (alternatively, rate of filtration) required to process a given amount of feed stream material.
Cross-flow filtration differs from dead-end filtration in that not all the fluid of the feed stream passes through the filter membrane and not all the contaminants are retained on the filter membrane. In cross-flow filtration, the feed stream of contaminated fluid is flowed along a surface of a filter membrane that is permeable to the fluid but impermeable to one or more contaminants in the fluid. Some of the fluid passes through the filter membrane, which removes some of the contaminants so that the fluid that emerges is purified. This purified fluid is called “permeate” or “product”. The remaining fluid by-passes the filter membrane, carrying contaminants with it, and exits without being filtered. This material is called “retentate” or “reject” or “concentrate.” The continual motion of fluid across the surface of the filter membrane causes some of the contaminant material to be removed from the filter membrane and to exit with the reject. The filter membrane is normally packaged in a flow-through vessel to provide a filter element to which a feed stream is provided and from which permeate and retentate are collected.
Cross-flow filtration can be used to filter smaller particles than is practical with dead-end filtration. Four general classes of cross-flow filtration are known in the art: microfiltration, ultrafiltration, nanofiltration and reverse osmosis. Each of these classes of filtration makes use of a semi-permeable filter membrane (sometimes also referred to herein simply as a “filter” or as a “membrane”). Cross-flow filter membranes are sometimes classified as low-pressure membranes (which include microfiltration membranes and ultrafiltration membranes) and high-pressure membranes (which include nanofiltration membranes and reverse osmosis membranes). Low-pressure membranes filter particulates including organisms such as bacteria and viruses. Nanofiltration membranes filter particulates and some larger ions. Reverse osmosis membranes filter particulates and a wider range of ions than does nanofiltration.
Each form of cross-flow filtration suffers from one or more problems which limit productivity. The “flux,” or productivity, of a membrane, i.e, the volume of water which may be processed by a membrane, decreases with increasing fouling. Fouling may consist of inorganic/scale, particulate/colloidal, microbial, or organic deposits that accumulate on the membrane. The productivity of a low-pressure membrane is limited by two primary factors, the growth of biological material (biofilm) on the membrane and the filterability of particulate matter. The filterability of the particulate matter is a function of the size of the particles and the degree to which they deform or pack as they are filtered. Large, hard, non-deforming particles are more easily retained by most filters without clogging than are small, sticky particles. In general, biofilm is the result of bacterial growth on surfaces, including the surfaces of filter membranes. A common problem for low-pressure membranes is that the fluid to be filtered (commonly, water) contains both bacteria and nutrients for the bacteria. The bacteria settle on the membrane and secrete a polysaccharide binder or glue. The presence of this film significantly reduces the permeability of the biologically coated membrane. Low-pressure membranes can be backwashed if they become clogged with particulates or similar contaminants.
Typically, a high-pressure membrane is mounted on, and wound around, a perforated product collection tube. The wound membrane-product collection tube assembly is mounted in a pressure vessel to yield a filter element. The pressure vessel has an input where a feed stream is flowed under pressure into contact with the membrane, and two outputs, one output being the product collection tube (where permeate is collected) and the other being a collection tube for fluid containing the contaminants that did not pass through the membrane (where retentate is collected). The filter element is constructed such that only the permeate can enter the product collection tube.
Most high-pressure membranes are packaged (wound) so that the spaces through which the fluid to be filtered must pass are very small. In addition, high-pressure membranes cannot be backwashed and must be protected from most if not all particulate matter by pretreatment steps. For example, it is common practice to provide pretreatment of feed stream water in a reverse osmosis system, to remove most of the particulate matter before the water passes to the reverse osmosis filter element. The pretreatment can be in the form of conventional filtration (sand filter, coagulant addition, settling) or in the form of a low-pressure membrane (with the inherent limitations described above). Pretreatment for a reverse osmosis system may also include pH regulation of the feed stream.
Despite such pretreatment, high-pressure membranes have performance limiting problems. The two primary fouling problems for high-pressure membranes are biofilm and scaling. The biofilm problem is as previously described for low-pressure membranes. Addressing biological problems in high-pressure membranes are more difficult than it is in low-pressure membranes because the spaces in which the biofilm grows are smaller, making plugging with live or dead bacteria more of a concern, and because high-pressure membranes are quite sensitive to biocides such as chlorine.
Scaling is a problem which can occur in nanofiltration membranes but which is much more prevalent in reverse osmosis membranes. In the process of reverse osmosis, water containing some very fine particulates and ions (e.g., Ca+2, Na+, Mg+2, Cl−, CO3−2, SO4−2) is passed along a first side of a membrane. As the feed stream water to be filtered passes along the membrane, some of the feed stream passes through the membrane and is emitted from a second side of the membrane as pure water (i.e., water that is substantially free of ions). This process causes the ions which remain on the first side of the membrane to increase in concentration in the remaining water, generating retentate. If sufficient water is allowed to pass through the membrane, the remaining solution (the retentate) will reach saturation for some of the ions present and minerals (e.g., CaCO3 and or CaSO4) will precipitate. This precipitate will inhibit, and may preclude, the further passage of water through the membrane. The need to avoid scaling limits the percentage of pure water which can be recovered from the feed stream.
A typical reverse osmosis filter element will yield about 50% permeate liquid, and 50% concentrate liquid. For example, if 100 gallons per minute (GPM) of liquid is fed into a reverse osmosis system, approximately 50 GPM of permeate will be output, and 50 GPM of concentrate or retentate will also be output.
A reverse osmosis system may have several stages wherein the retentate from one filter element (the first stage) is processed by a subsequent filter element (a second stage), and so on. In general, a maximum recovery from the initial feed stream is about 85% converted to permeate. Accordingly, when 100 million gallons of feed liquid is processed through the stages of a reverse osmosis system, approximately 15 million gallons of retentate will be generated and must be properly disposed of.
As noted above, after a period of operation, a reverse osmosis membrane can become fouled with mineral scale and/or bacteria that diminish the filtration performance of the membrane. Bacteria contribute to the formation of a biofilm that further diminishes the performance of the membrane. Typically, when a membrane in a reverse osmosis system becomes fouled to an unacceptable level, the membrane must be removed from use to be cleaned. However, each time a membrane is cleaned, its effectiveness and production capacity is reduced.